3.4 Rapid Assessment of Fluid Responsiveness via Cardiopulmonary Interactions

Fluid responsiveness assessment is the core of volume management. Since pulse pressure variation (PPV) and stroke volume variation (SVV), efforts to expand the application scope of preload dynamic indices based on cardiopulmonary interaction have continuously explored new indices and methods. This article discusses two novel cardiopulmonary interaction-based dynamic indices/methods from 2025: ΔPP after sigh maneuver (ΔPP-SIGH₁₅ₛ) and ΔPP after 10 s end-expiratory occlusion (ΔPP-EEO-10s).

1. Bottlenecks in Predicting Fluid Responsiveness Based on Cardiopulmonary Interaction

Preload responsiveness dynamic indices simulate volume challenge effects through cyclical pulmonary blood volume changes from inspiration/expiration, thereby predicting fluid responsiveness. Application requires: ① Regular mechanical ventilation generating sufficient intrathoracic pressure fluctuations; ② No interference from spontaneous breathing efforts. When using PPV/SVV, patients typically require controlled ventilation with fixed tidal volumes of 8~12 ml/kg ideal body weight (IBW), which is a major clinical bottleneck. In ARDS low tidal volume ventilation, the >8 ml/kg IBW condition is unmet, potentially yielding non-significant PPV/SVV changes despite existing fluid responsiveness.

To address this, Myatra et al. proposed the “tidal volume challenge” in 2017: measuring PPV/SVV changes after transiently increasing tidal volume from 6 to 8 ml/kg IBW for 1 min to predict fluid responsiveness. It effectively improves prediction accuracy during low tidal volume ventilation but remains inapplicable to other PPV/SVV limitations like spontaneous breathing, arrhythmias, open chest, elevated intra-abdominal pressure, and RV dysfunction.

Messina et al. proposed in 2019 using PPV after ventilator-generated sigh maneuvers to predict fluid responsiveness in pressure support ventilation (PSV) patients. In 40 patients, a >35% PPV drop from baseline predicted fluid responsiveness with AUC = 0.91 (0.82~0.99), sensitivity 75.0%, specificity 91.6%.

End-expiratory occlusion test (EEOT) and subsequent combined end-expiratory and end-inspiratory occlusion test (EEOT+EIOT) were also proposed. EEOT pauses at end-expiration for 15 s, simulating increased venous return; if CO increases, fluid responsiveness is indicated. However, EEOT’s CO variation threshold is small, potentially increasing measurement error. A 15 s apnea may cause patient discomfort or inspiratory effort, and increased transpulmonary pressure may raise RV afterload, raising safety concerns in moderate-to-severe ARDS or acute pulmonary heart disease. Jozwiak et al. later proposed EEOT+EIOT, significantly raising the diagnostic threshold to 13%, improving clinical diagnostic performance.

Against this backdrop, researchers proposed ΔPP-SIGH₁₅ₛ and ΔPP-EEO-10s to further enhance cardiopulmonary interaction-based fluid responsiveness prediction capability.

2. ΔPP-SIGH₁₅ₛ: Preload Testing Under Active Intervention and Neural Reflex Regulation

SIGH₁₅ₛ involves the ventilator actively delivering a brief, high-intensity, standardized “sigh” (35 cmH₂O, lasting 4 s), simulating a reproducible acute preload reduction event. Effects include: ① High-pressure sigh instantly elevates intrathoracic pressure, significantly reducing venous return and RV preload; for fluid responders on the Frank-Starling ascending limb, stroke volume drops markedly; ② Stimulates pulmonary stretch receptors, activating vagus-mediated Hering-Breuer reflex, inhibiting brainstem inspiratory centers, suppressing inspiration, and prolonging expiration, maximizing reduction of inspiratory effort impact on RV preload. This effect not only reduces spontaneous breathing interference on venous return during the measurement window but may create a brief hemodynamic “steady-state” period, allowing observed PP changes to be more purely attributed to stroke volume changes from preload reduction.

Messina et al.'s 2025 new study further validated its utility. The prospective study enrolled 56 PSV ICU patients requiring volume expansion, using >10% CO increase after 4 ml/kg crystalloid within 10 min as the fluid responsiveness reference standard, comparing ΔPP-SIGH₁₅ₛ with ΔCO-EEOT reliability. Results showed 27 responsive and 29 non-responsive patients. ΔPP-SIGH₁₅ₛ AUC for distinguishing fluid responsiveness was 0.93 (optimal threshold ΔPP-SIGH₁₅ₛ drop ≥25%, sensitivity 93.1%, specificity 91.6%), superior to ΔCO-EEOT’s 0.67; but in low inspiratory effort (P₀.₁ ≤ 1.5 cmH₂O) subgroup, both performed similarly (AUC 0.98 vs. 0.89).

Advantages of ΔPP-SIGH₁₅ₛ Study: ① Maintains excellent diagnostic performance with significant spontaneous breathing effort, outperforming EEOT, solving the technical bottleneck of traditional dynamic indices during spontaneous breathing; ② Intervention implementation via built-in ventilator function, short duration (4 s), good patient tolerance; ③ “Stimulus–response” model reduces respiratory drive and neural reflex interference.

Limitations of ΔPP-SIGH₁₅ₛ Study: ① Optimal threshold is −25%, but diagnostic gray zone is −15%~−35%, a wide range; ② Safety in specific pathological states requires further study (e.g., bullae, severe heart failure, severe ARDS, tracheoesophageal fistula, intracranial hypertension); ③ In patients with weak respiratory effort (P₀.₁ < 1.5 cmH₂O), predictive performance matches simpler EEOT, showing no superior performance. Thus, SIGH₁₅ₛ’ core value should be positioned for predicting fluid responsiveness in significant spontaneous breathing scenarios.

3. ΔPP-EEO-10s: Apnea Time Optimization for Breath Occlusion Tests

Gavelli et al. made two key modifications to classic EEOT: ① Time optimization, shortening EEOT apnea from standard 15 s to 10 s, further exploring 5 s feasibility; ② Index simplification, using PP change (ΔPP) instead of cardiac index change (ΔCI) as the observation endpoint.

The study enrolled 143 sinus rhythm controlled ventilation patients, recording ΔPP and ΔCI during 15 s EEO and EIO, continuously monitoring and comparing diagnostic value across time intervals (0~5 s, 5~10 s, 10~15 s) for predicting preload responsiveness. Preload responsiveness defined as PLR-induced ΔCI ≥ 10% or fluid challenge-induced ΔCI ≥ 15%. 61 responsive and 82 non-responsive patients enrolled.

ΔPP-EEO-10s Study Results:

  1. ΔCI: ΔCI-EEO-10s and ΔCI-EEO-15s AUC for detecting fluid responsiveness showed no significant difference (0.910 vs. 0.935, P = 0.801), optimal threshold 2.1%; ΔCI-EEO-5s failed to effectively detect fluid responsiveness. Additionally, ΔCI-EEO-15s (AUC = 0.935) and ΔPP-EEO-15s (AUC = 0.913) showed no significant difference.

  2. ΔPP: ΔPP-EEO-10s AUC was 0.912, sensitivity 86.9%, specificity 80.5% at optimal threshold (3.7%); no difference from ΔPP-EEO-15s AUC, but significantly higher than ΔPP-EEO-5s AUC. Core result: ΔPP-EEO-10s AUC is comparable to the “gold standard” ΔCI-EEO-15s.

  3. EEO + EIO Combined: Similar results observed; ΔPP-EEO+EIO-10s and ΔPP-EEO+EIO-15s AUC showed no significant difference, but both significantly higher than ΔPP-EEO+EIO-5s.

Advantages of ΔPP-EEO-10s Study: ① Confirms EEO shortened from classic 15 s to 10 s maintains excellent diagnostic performance, indicating high feasibility of EEO-10s; ② Uses easily obtainable ΔPP instead of complex equipment-required ΔCO/ΔCI as observation endpoint, lowering technical barriers and equipment dependence.

Limitations of ΔPP-EEO-10s Study: ① ΔPP-EEO-10s does not fully solve assessment challenges in spontaneous breathing patients, only applicable to controlled ventilation scenarios; ② Measurement accuracy highly depends on high-quality arterial pressure waveforms; ③ Study data is retrospective, determined diagnostic thresholds require prospective validation.

4. Summary

In summary, ΔPP-SIGH₁₅ₛ and ΔPP-EEO-10s suit different clinical scenarios and offer optimized improvements over existing methods. ΔPP-SIGH₁₅ₛ suits patients with spontaneous breathing, advantageous in reducing spontaneous breathing interference, but disadvantaged by relatively complex operation, specific contraindications, and a “gray zone.” ΔPP-EEO-10s primarily suits patients without spontaneous breathing under full controlled ventilation, simplifying the classic 15 s EEOT and enhancing clinical convenience.

(Gong Chao, He Huaiwu, Peking Union Medical College Hospital)

References

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